1. Field
The present invention relates to an apparatus and method of sensing a sleeping condition of a user, and more particularly, to a sleeping condition-sensing apparatus and method in which it is determined that a user who is sleeping moves into an awake state when his or her physiological signal is measured and a certain motion artifact is sensed from the measured physiological signal.
2. Description of the Related Art
Sleep provides human beings with essential resting time for relaxation and it is through sleep that the supply of energy is carried out to reserve physical power. Sleep is the time period during which human beings supplement energy consumed while doing activities during the day time. Further, human growth hormone (HGH) indispensable for children in a growth phase is secreted in the greatest amount just during the sleep period. The brain of our body is a complex organ that collectively controls all the physiological functions for sustaining the human life and physical rest is necessary for the brain to maintain a suitable balance of its activities. This bodily rest is primarily taken during the sleep period.
As such, although one cannot emphasize too much the importance of sleep, most modern people do not sleep sufficiently due to excessive loads of business, schoolwork or the like. Most important in sleep is the quality of sleep, rather than the quantity of sleep. Particularly, whether or not people take a deep sleep is significantly crucial, however almost all modern people do not maintain a good quality of sleep at night.
In order to get a good quality of sleep for the rest at night, it is required that after an individual person has determines the quality of his or her sleep, he or she should seek measures to cope with its result. There are many occasions where all people do not exactly recognize the quality of their sleeps in their real lives as in the case where a person does not feel refreshed but is still tired despite a long sleep, i.e., in the case where the person feels the person sleeps well but is easily tired, or in the case where the person thinks the person tossed and turned all night long but actually has no problem with the person's sleep itself, etc. This falls within a kind of problem that can be accurately identified only when the ordinary sleeping condition of an individual is measured and digitized by means of an objective measurement instrument. Even when there is no case of such a distinct problem, the optimum sleep time for each individual will be able to be derived by digitizing the quality of his or her sleep. There is a need for development of a simple measurement instrument to determine the quality of an individual's sleep. But, up to now, polysomnography (PSG), which is performed in a sleep lab of a hospital, is a unique objective method of diagnosing and quantifying the quality of sleep or the severity of sleep disordered breathing (SDB).
A human sleep classified based on the PSG is described as a succession of five recurring stages including four non-rapid eye movement (NREM) stages and a rapid eye movement (REM) stage. During a normal sleep period at night, a person usually periodically goes through a sequential sleep cycle of stage 1, 2, 3, 4, 3, 2 and REM in a period of approximately 90 minutes. The sequence of such a sleep cycle is shown in FIG. 1.
The determination of a subject's sleeping condition is performed by an expert who has integrated various measuring signals obtained from PSG while he or she sleeps in a hospital lab at night. Sleeping condition, obstructive sleep apnea (OSA), arrhythmia, rapid eye movement, etc., are diagnosed and evaluated by synthesizing various kinds of signals. The quality of sleep is determined depending on how a subject is frequently awakened from sleep or frequently moves into deep-sleep stages 3 and 4 during the sleep period.
The signals measured during PSG includes signals associated with electroencephalogram (EEG), electro-oculogram (EOG), electromyogram (EMG), electrocardiogram (ECG), arterial oxygen saturation (SaO2), chest and abdomen respiratory effect, nasal airflow, snoring, body temperature, body position changes and recorded video for identifying the subject's tossing and turning during sleep, and the like. Since these various special instruments must be used for PSG, the subject suffers from an inconvenience of having to sleep in a sleep lab of a hospital and the cost for polysomnography is increased. In addition, PSG requires transformation of a sleeping environment into a hospital sleep lab, but not a place where a person usually sleeps so that a sleeping subject may exhibit a different characteristic of sleeping condition than that of ordinary times. Furthermore, a variety of measurement instruments are respectively attached to head, eye peripheries, chin peripheries, chest, abdominal region, etc., of a sleeping subject for the purpose of measurement of various signals, so that inconvenience and displeasure are is given to him or her, and he or she also shows a different characteristic of sleeping condition than that of ordinary times, making it difficult to diagnose this sleeping condition as a normal one.
Therefore, efforts are required to reduce the number of measurement instruments to be attached to a sleeping subject to increase convenience of use, to build a sleeping environment more similar to an ordinary one and with reduced costs, and to limit an attaching position of the measurement instruments to bodily regions excluding the head or face. Research is being conducted on diagnosis of sleeping condition using only an ECG signal as part of these efforts. In this case, ECG signals are processed based on heart rate variability (HRV) so as to extract low-frequency components and high-frequency components from the signals to thereby determine an awake stage, stages 1, 2, 3, 4 and REM. However, in the case of the HRV-based sleeping condition diagnosis, a degree of dispersion is increased and hence a differentiation between respective sleep stages is impossible.
When only awake stages and the remaining sleep stages are differentiated without accurately distinguishing all the awake stages, stages 1, 2, 3, 4 and REM from one another, the quality of sleep can be assessed. The frequency of awakenings during the PSG test is used as an index for determining the quality of a subject's sleep. As shown in FIG. 1, sufficient time is spent to go into deep-sleep stages 3 and 4. In this case, recurrent awakenings from sleep means a state where a subject did not fall into deep-sleep phases as often, and the period of stages 3 and 4 and the frequency of awakenings are inversely proportional to each other.
A conventional prior technique for assessing the quality of sleep employs electrical measurement, optical measurement, ECG, galvanic skin response (GSR), etc. In the case of the electrical measurement method, a motion artifact is generated due to electrodes themselves attached to a skin in the region of a soft body tissue and a skin tissue epidermis and a change in the electric properties (e.g., impedance) between the electrodes by a subject's movement. Also, in the case of the optical measurement method, a motion artifact is generated due to a skin tissue itself and a change in the optical properties between the skin tissue and a light source by a subject's bodily motion. In addition, in the case of the ECG or GSR method, when a physiological signal of a subject is measured, a myo-electric signal generated due to his or her bodily motion also falls within a kind of motion artifact. Such a motion artifact is regarded as a noise signal and becomes a target to be eliminated in the case of measuring a physiological signal of a sleeping subject for the purpose of assessment of the quality of his or her sleep. Then, in order to sense the subject' being tossed and turned during the sleep, a method of recording his or her images using a camera to diagnose the quality of sleep, and a method of attaching a separate motion sensor such as an accelerometer to the sleeping subject to identify his or her motion.
However, in assessment of the quality of sleep, REM stage sleep is important, but the frequency of awakenings during the sleep is an insignificant factor. Therefore, there is an urgent need for an apparatus for and method of sensing a sleeping condition in which the quality of sleep can be more easily and simply determined based on a motion artifact signal without an additional measurement instrument.